What Mako robotic-arm surgery does before and during the operation

Mako robotic-arm surgery operates in two distinct stages, and understanding both is what separates the technology's genuine benefit from the marketing shorthand of 'robotic precision'.

Before the operation, a CT scan of the patient's hip generates a three-dimensional digital model specific to that individual's anatomy — not a population average or a shared template. Working from this model, the surgeon plots the exact angle, depth, and rotation of the acetabular cup, alongside femoral offset and any leg-length correction required, to within approximately 0.5 mm. Every target is set and agreed before the first incision is made.

During the operation, the robotic arm enforces those pre-agreed targets through haptic boundaries — a form of physical force-feedback that halts or resists the instrument the moment it approaches the edge of the planned resection zone. This is mechanical compliance with the plan, not advisory guidance the surgeon may choose to ignore: the arm will not permit bone removal outside the area mapped pre-operatively. A practical analogy is guard rails on a narrow road — the driver (the surgeon) still controls direction, speed, and clinical judgement at every step; the rails prevent the vehicle from crossing a boundary that should not be crossed.

The surgeon remains fully in charge throughout. Mako constrains execution to a carefully constructed patient-specific plan; it does not make decisions autonomously, and it does not replace the consultant's intraoperative judgement when unexpected anatomy or tissue condition requires adaptation.

Why cup position matters so much in hip arthroplasty

Millimetres turn out to matter a great deal once a prosthetic cup is fixed inside the pelvis and cannot be adjusted without further surgery.

The acetabular cup — the socket half of the hip replacement — must sit within a narrow band of angles to function correctly. The widely used Lewinnek safe zone defines this as roughly 40° (± 10°) of inclination and 15° (± 10°) of anteversion. Stray outside those limits and the bearing surface wears unevenly, the risk of the femoral head slipping out of the socket rises, and leg-length equality becomes harder to achieve.

In conventional manual surgery, outlier placement is common rather than exceptional. Comparative data from large published series show that approximately one in three cups falls outside the Lewinnek parameters when positioned by hand — however experienced the surgeon. The problem is not surgical competence; it is that soft tissue, patient positioning, and intraoperative visual cues alone cannot reliably reproduce a sub-millimetre target set pre-operatively on paper.

The consequences of dislocation are not a single manageable event. Published data indicate that 57% of patients who dislocate do so more than once, and 45.6% require revision surgery within two years. Revision carries substantially greater surgical risk than the original procedure.

Cup position is, in this sense, the most controllable intraoperative variable in hip arthroplasty — and the one whose downstream effects, from bearing wear to revision risk to gait symmetry, accumulate over the lifetime of the implant.

What the evidence shows for Mako-assisted hip replacement

A 2025 systematic review and meta-analysis of 20 comparative studies found that Mako-assisted total hip arthroplasty placed 94.7% of acetabular cups within the Lewinnek safe zone — against 65.8% for conventional manual surgery — and 90.3% within the tighter Callanan zone, versus 57.1% by hand. Patient-reported outcomes also favoured Mako: Forgotten Joint Scores were significantly higher (weighted mean difference +8.7, p = 0.005) and Oxford Hip Scores meaningfully better (+1.5, p = 0.03). The Forgotten Joint Score is a particularly useful measure here, since it captures the proportion of daily activity during which the patient is unaware of their hip — a direct patient-experience outcome rather than a clinician-rated functional score.

A separate meta-analysis of 38 studies covering more than 10,000 patients confirmed both the positioning advantage and a lower complication rate (p = 0.04) in the robotic group. That is a meaningful safety signal. However, the same analysis found no statistically significant difference in Harris Hip Score, WOMAC, or revision rates — an important qualification that warrants plain statement rather than footnote treatment.

A network meta-analysis of 45 randomised controlled trials goes further: no robotic or navigation modality has yet demonstrated a significant reduction in revision surgery compared with conventional technique. The evidence therefore supports Mako as a precision tool that substantially reduces positional outliers and improves early patient-reported outcomes. Whether that translates into fewer revisions at ten or fifteen years remains an open question, because the long-term data do not yet exist.

Positional accuracy is, in other words, necessary but not sufficient on its own. The stabilising architecture of the soft tissue around the replaced joint — the tendons, proprioceptive structures, and passive restraints — is where SPAIRE adds a second, independent layer of protection.

How SPAIRE's tendon preservation works alongside robotic precision

That second layer rests on two anatomical mechanisms active from the moment the wound is closed.

SPAIRE — Save Piriformis And Internus, Repair Externus, first described by Kim et al. in 2008 — modifies the standard posterior approach by leaving the piriformis and obturator internus tendons intact rather than detaching and reattaching them. The obturator internus runs directly over the posterior femoral head, near the centre of rotation. Kept in place, it acts as a natural seatbelt across the back of the joint: a passive 'strap effect' whose viscoelastic tension resists posterior subluxation from the outset, without waiting for repaired tissue to regain its strength.

Preservation also keeps the nerve endings within those tendons — Golgi tendon organs and muscle spindles — continuously active. In a standard posterior approach, detachment severs this afferent signalling; the hip loses the spatial feedback it sends to the brain until neural regeneration catches up. SPAIRE maintains that proprioceptive continuity from day one, which may contribute to the early mobility and low dislocation rates recorded in published cohort data.

The practical trade-off is visual. Limiting soft-tissue disruption also limits the surgeon's direct line of sight to the acetabulum, and cup positioning that depends on anatomical landmarks alone carries the same potential for positional error as any manual technique. Mako's haptic boundaries resolve this tension directly: planned cup angles are mechanically enforced regardless of how constrained the exposure, while the intact posterior structures simultaneously provide tactile feedback for femoral offset and leg-length judgement. The combination addresses two distinct failure modes — positional error and early soft-tissue instability — rather than offering the same protection twice, though direct comparative outcome data for Mako used specifically with SPAIRE have not yet been published.

SPAIRE compared with other approaches to hip arthroplasty

Choosing a surgical approach is a separate decision from choosing whether to use robotic assistance, and it depends on anatomy, bone morphology, prior surgery, and surgeon expertise as much as on the evidence from population-level studies.

Standard posterior approach is the most widely performed technique in the UK. It offers broad anatomical exposure and a well-established long-term safety record. Historically it carried a higher short-term dislocation rate than anterior routes, largely because the posterior capsule and external rotator tendons were detached — though capsular repair techniques have reduced this risk in experienced hands. Long-term revision data for the standard posterior approach are more mature than for any of the alternatives described here.

Lateral (Hardinge) approach avoids the posterior dislocation risk by not disturbing the posterior structures, but it requires partial detachment of the abductor muscle group — the same muscles that power the sideways push-off phase of walking. A small proportion of patients develop a persistent Trendelenburg gait (a dipping of the pelvis on the operated side) as a result, and the approach provides no equivalent of SPAIRE's posterior strap effect.

Anterior/DAA is muscle-sparing from the front and avoids all posterior structures entirely. Cohort data suggest earlier pain relief and faster early functional return than posterior routes, but the technique typically requires specialist operating-table equipment, fluoroscopic imaging, and carries a distinct nerve risk — neurapraxia of the lateral femoral cutaneous nerve is well documented. The learning curve for surgeons is steeper, and blood loss can be higher.

SPAIRE sits between the standard posterior and DAA in terms of soft-tissue disruption. The 2025 cohort study of 858 cases found significantly better mobility scores at three months versus the direct lateral approach, and a 194-case series recorded zero dislocations in the SPAIRE group. However, long-term equivalence with conventional approaches has not yet been confirmed in randomised data; the multicentre HemiSPAIRE RCT is ongoing, and its results are needed before any definitive claim about long-term outcomes can be made.

SPAIRE is not appropriate for every patient. Significant anatomical variation, high BMI, previous hip surgery, or specific bone morphology may make a different approach the safer or more reproducible option. Which technique is recommended depends on a consultant-led assessment of individual circumstances — not a single-approach policy.

Patient suitability, assessment, and what to expect at consultation

Suitability for Mako-assisted SPAIRE is not universal, and a consultant-led assessment is the appropriate place to establish whether both techniques are warranted for a specific patient.

The key variables at assessment are bone morphology and acetabular anatomy — which determine whether robotic pre-planning adds meaningful precision — alongside soft-tissue condition, BMI, any previous hip surgery, and the operating surgeon's training. Not every centre offering Mako robotic assistance also performs the SPAIRE approach, and not every centre performing SPAIRE has access to robotic planning; patients benefit from asking explicitly which combinations are routinely available.

A thorough pre-operative assessment typically includes clinical examination, functional scoring, and imaging. Where Mako planning is indicated, a CT scan is acquired to build the patient-specific 3D model used for cup position, femoral offset, and leg-length planning. The consultation should also include an open discussion of approach options — as outlined in the preceding section, SPAIRE is not appropriate for every anatomy, and a team experienced across multiple approaches is better placed to make that judgement than one with a fixed single-approach policy.

Patients can reasonably ask which surgical approaches the team performs routinely, how intraoperative cup position is verified, and what the process looks like if the pre-operative plan needs adjustment during surgery.

Professor Paul Lee, whose clinical practice integrates the SPAIRE muscle-sparing posterior technique with objective biomechanical assessment before and after surgery, consults at centres in Sleaford and Grantham, Lincolnshire. Hip Replacement Lincolnshire is part of the MSK Doctors group and accepts patients without a GP referral; assessments can be booked at hipreplacementlincolnshire.co.uk.

1. [1] Comparison of Implant Placement Accuracy Between Manual, Robot-Assisted, Computer-Navigated, Augmented Reality Navigated, Patient-Specific Instrumentation, and Accelerometer Navigated THA. (2024). https://doi.org/10.2106/JBJS.RVW.24.00120 https://doi.org/10.2106/JBJS.RVW.24.00120
2. [2] Robotic-Assisted THA Provides Greater Implant Placement Accuracy and Lower Complication Rates, But Not Superior Clinical Results Compared to Conventional Manual Approach: A Systematic Review and Meta-Analysis. (2024). https://doi.org/10.1016/j.arth.2024.12.014 https://doi.org/10.1016/j.arth.2024.12.014
3. [3] MAKO robotic-assisted compared to conventional total hip arthroplasty for hip osteoarthritis: a systematic review and meta-analysis. (2025). https://doi.org/10.1186/s13018-025-05866-1 https://doi.org/10.1186/s13018-025-05866-1